1. Field of the Invention
The present invention relates to an alkaline promoter system comprising specific mixtures of non-sterically hindered amino compounds and sterically hindered amino acids and their use in acid gas scrubbing processes, particularly in the "hot pot" type acid gas scrubbing processes.
2. Description of the Related Patents
The present invention pertains to an improved process for carrying out what is known as the aqueous base scrubbing process or "hot potash" ("hot pot") process. In this process a relatively small level of an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is desired. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions and an amine activator. Many industrial processes for removal of acid gases, such as CO.sub.2, use regenerable aqueous alkaline scrubbing solutions, such as a potassium carbonate and an activator comprising an amine, which are continuously circulated between an absorption zone where acid gases are absorbed and a regeneration zone where they are desorbed, usually by pressure reduction and steam-stripping. The capital cost of these acid gas scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance. The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbents which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
There are a number of patents which describe processes to improve the efficiency of the "hot potash" process. Some of these improvement processes are described below.
In U.S. Pat. No. 2,718,454, there is described a process for using potash and similar alkali metal salts in conjunction with amines, such as monoethanolamine, diethanolamine and triethanolamine to remove acid gases from a gas mixture. The combination of the alkali metal compounds in conjunction with the designated amine yields higher capacity for acid gases than systems with the amines alone.
In U.S. Pat. No. 3,144,301, there is disclosed the use of potassium carbonate in conjunction with monoethanolamine and diethanolamine to remove CO.sub.2 from gaseous mixtures.
U.S. Pat. Nos. 3,563,695; 3,563,696, and 3,642,430 to Benson et al. disclose processes for removing CO.sub.2 and H.sub.2 S from gaseous mixtures by alkaline scrubbing processes wherein at least two separate regeneration zones are provided. Alkanolamines and amino acids such as glycine are described as activators, but the use of sterically hindered amino compounds is not taught or disclosed in these patents.
In U.S. Pat. Nos. 3,637,345; 3,793,434, and 3,848,057 processes for the removal of acid gases by means of aqueous carbonate scrubbing solutions activated by an amino compound such as 1,6-hexanediamine, piperidine and their derivatives are described.
In U.S. Pat. No. 3,856,921, there is disclosed a process for removal of acid gases from fluids by use of a basic salt of an alkali or alkaline earth metal and an amino compound activator, such as 2-methylaminoethanol, 2-ethylaminoethanol, morpholine, pyrrolidine and derivatives thereof.
Belgian Pat. No. 767,105 discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a solution comprising potassium carbonate and an amino acid, such as substituted glycines (e.g., N-isopropyl glycine, N-t-butylglycine, N-cyclohexylglycine, etc.). The data in Table IV of the patent indicates that the highly substituted compounds, such as N-t-butylglycine, are inferior to the straight chain compounds, such as N-n-butyl glycine, but N-cyclohexyl glycine, a sterically hindered amine, has a good rate of absorption. Similarly, British Pat. No. 1,305,718 describes the use of beta- and gamma amino acids as promoters for alkaline salts in the "hot pot" acid gas treating process. These amino acids, however, are not suitable because the beta-amino acids undergo deamination when heated in aqueous potassium carbonate solutions. The gamma amino acids form insoluble lactams under the same conditions.
Recently, it was shown in U.S. Pat. No. 4,112,050 that sterically hindered amines are superior to diethanolamine (DEA) and 1,6-hexanediamine (HMDA) as promoters for alkaline salts in the "hot pot" acid gas scrubbing process. U.S. Pat. No. 4,094,957 describes an improvement to the '050 patented process whereby amino acids, especially sterically hindered amino acids, serve to prevent phase separation of the aqueous solution containing sterically hindered amines at high temperatures and low fractional conversions during the acid gas scrubbing process. In these patents "sterically hindered amines" are defined as amino compounds containing at least one secondary amino group attached to either a secondary or tertiary carbon atom or a primary amino group attached to a tertiary carbon atom. At least one nitrogen atom will have a sterically hindered structure.
In some instances, where an existing commercial gas treating plant utilizes a non-sterically hindered amine promoter such as diethanolamine or 1,6-hexanediamine, there is a need to increase the CO.sub.2 scrubbing capacity due to increased levels of CO.sub.2 in the gas. The need to meet this increased CO.sub.2 capacity can be accomplished by increasing the size of the plant (e.g., adding treating towers and the like) or by replacing the non-sterically hindered amine with sterically hindered amines as proposed in U.S. Pat. Nos. 4,094,957 and 4,112,050. In the case of the latter approach, the preexisting scrubbing solution must be removed and replaced with the fresh solution containing potassium carbonate and sterically hindered amine. This change-over procedure requires some "down-time" of the plant with consequent losses of production. Therefore, increasing the size of the gas treating plant or changing over the scrubbing solution can be costly.
It has now been discovered that one may add sterically hindered amino acids to the non-sterically hindered amino compound-promoted carbonate scrubbing solution and thereby increase the CO.sub.2 absorption rate relative to that of the pre-existing non-sterically hindered amino compound-promoted carbonate.